You should keep a list of the top ten risks that can impact your project.

Probably should read: "top ten risks that can impact your project, and that the project team is capable of managing. There are many risks external to the project that can nonetheless impact it (like the company going out of business) that may be beyond the team's ability to manage.

In practice, ten may be too many. Keeping a list of the top five may be enough. I'm not suggesting that a project won't have ten important risks, but the longer your list is, the less focused you'll be on addressing each item. --KrisJohnson

In my practice, keeping track of the top two would have been enough. Usually when something goes badly wrong it's something that could have that easily been predicted--even if I didn't. --MarkSchumann